Gravitationally lensed radio emission associated with SMM J16359+6612, a multiply imaged submillimeter galaxy behind A2218

0.19 MB·English

by  M. A. Garrett

Astronomy & Astrophysics manuscript no. Gl071˙astro-ph February 2, 2008
(DOI: will be inserted by hand later)
Gravitationally lensed radio emission associated with SMM
J16359+6612, a multiply imaged submillimeter galaxy behind
A2218
1 2 3
M.A. Garrett , K.K. Knudsen & P.P. van der Werf
5 1 Joint Institutefor VLBI in Europe, Postbus 2, 7990 AA,Dwingeloo, The Netherlands
0 2 Max-Planck-Institutefu¨r Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany
0 3 University of Leiden, Department of Astronomy,P.O. Box 9513, 2300 RA Leiden, Netherlands.
2

n Received ...; accepted ...
a
J
Abstract. An abstract should be given
6

1
v 1. Introduction radio counter-part is estimated to be only a few microJy
5 at 8.2 GHz. However, the magnification provided by the
9 SMMJ16359+6612 is a sub-millimetre galaxy (SMG) lo- lens boosts the flux density of even the faintest image in
0 cated at z=2.516 that is triply imaged and highly mag-
SMMJ16359+6612 by a factor of 9, suggesting that ra-
1 nified by the core of a massive foreground cluster, Abell
dio counterparts to the sub-mm sources associated with
0 2218(Kneibetal.2004a,Knudsen2004).Thethreelensed
5 SMMJ16359+6612 should be easily detectable in deep
imagesofthe backgroundsourcearehighlymagnified(by
0 VeryLargeArray(VLA)andWesterborkSynthesisRadio
factorsof22,14&9)andthe maximumimageseparation
/ Telescope(WSRT)imagesofthefield.Wenotethatsingly
h is ∼ 41 arcseconds. All three images are detected by the
p imaged SMG, modestly amplified by foreground clusters,
SubmillimeterCommon-UserBolometricArray(SCUBA)
- havealreadybeendetectedatradiowavelengthse.g.Smail
o at 850 & 450 microns with the brightest image having a
et al. (2000) and Ivison et al. (2001).
r measured 850 micron flux density of 17 mJy (Kneib et
t Inthis paper wepresentWSRT 1.4GHz andVLA 8.2
s al. 2004a).Recently, both Kneib et al. (2004b)and Sheth
a GHz observations of Abell 2218, specifically the area of
: et al. (2004) have observed molecular emission from the skysurveyedbytheLeiden-SCUBAlenssurvey(Knudsen
v
CO(3-2) line for each of the three images. There are two
i 2004)thatincludesSMMJ16359+6612.TheVLAdataare
X distinct velocity components in this line separatedby 280
in the public domain and were extracted from the on-line
r kms/s, suggesting a total dynamical mass of the galaxy VLA archive. In section 2 we describe the radio obser-
a of 1.5×1010M⊙. There is also a spatial offset of 1 arcsec vations and data analysis. The results and discussion are
between these two components and Kneib et al. (2004b)
presented in section 3 and we give our conclusions in sec-
argue, that the source is likely to be a merging system
tion 4. Throughout this paper we assume an Ω = 0.3,
with the 2 nuclei separated by 3 kpc. Λ=0.7 cosmology with Hø =70 km s−1 Mpc−1.
At radio wavelengths only the bright tail of the SMG
populationis usuallydetectable,soobservationsofhighly
2. Radio Observations
magnified systems such as SMMJ16359+6612, offer a
unique opportunity to study the radio properties of one 2.1. WSRT Observations
example of the faint sub-mJy, SMG population. Little is
known about the nature of these faint SMG, despite the ObservationsofAbell2218at1.4GHz wereconductedby
fact that they dominate (energetically) the cosmic far- the WSRT on 5 November 2004. The 12 hour run used
infrared background (Knudsen 2004). The intrinsic (un- the full 160 (8 × 20) MHz WSRT observing band and
lensed)sub-mmfluxofSMMJ16359+6612isestimatedto employed the default continuum frequency set-up (with
be S850 = 0.8 mJy (placing it well below the confusion the eight bands centered between 1.311 and 1.450 GHz).
limit of normal blank field sub-mm surveys). If one as- Observationsweremadeassumingacoordinateequinoxof
sumes that the FIR-radio correlationholds for this high-z 2000. For each of the 8 bands, 64 spectral channels were
source(e.g.Garrett2002),theintrinsicfluxdensityofany generated (a total of 512 spectral points were obtained
for the 160 MHz band) and 4 polarization products were
Send offprint requests to: [email protected] recorded.Unfortunately,radiotelescope“RT7”wasbeing
2 Garrett et al.: Lensed radio emission in SMM J16359+6612
used for single-dish VLBI observations and was unavail-
ableduringthe entiretyoftheseobservations.Ashort20-
minutescanon3C48wasusedtoamplitudecalibratethe
data.Aphasereferencesource(J1642+689)wasobserved
for5minuteseveryhour.Thedataanalysiswasperformed
using the NRAO AIPS package. The fringe-fitted phase
solutions from J1642+689were applied to the A2218 tar-
get field. For this field, the entire WSRT primary beam
was imaged using the AIPS task IMAGR. Several bright
sources were detected in the field of view, and the data
were self-calibrated using these and other sources in the
field as a sky model. These outlying sources were later
subtracted from the data set and self-calibration contin-
ued using only those sources located in the centre of the
field.Thedatawereself-calibratedfirstinphaseandlater
inbothamplitudeandphaseusingtheAIPStaskCALIB.
A uniformly weighted (“robust -1”) WSRT image of
the field convolved with a Gaussian restoring beam of
13.6×12.4 arcseconds (in position angle, PA = −0.21◦)
is shown in Figure 1. At 1.4 GHz there is a great deal
of extended emission in the vicinity of the A2218 cluster
core. In order to enhance the contrast of the WSRT im-
Fig.1. The WSRT 1.4 GHz contour map superimposed
age,spacingsshorterthan800λwerenotusedtoformthe
uponanHSTF702Wimageoftheregionofskyassociated
imagepresentedinFigure1.The imagereachesa1σ rms
with SMM J16359+6612. Radio emission from all three
noise level of ∼15 µJy beam−1.
lensedimages(SMM-A,SMM-BandSMM-C)isdetected
bytheWSRT.TheboxesrepresenttheSCUBAsourcepo-
sitions. Radio emission from three other SCUBA sources
2.2. VLA observations
in the field (including the highly magnified arclet #289,
SMMJ16359+66118)arealsoboxed.Contoursaredrawn
at -3,3,5,10,20 & 40 times the 1-σ noise level of 15 mi-
We searched the VLA archive for observations associated
croJy/beam.
with the central area of Abell 2218. Several data sets
wereidentifiedthat includedshort,snapshotobservations
of this field. Only one full-track 8.2 GHz observation of
3. Results & Discussion
A2218 was identified, AR045. These X-band observations
were made on 1999 June 18 UT with the VLA in its D (3 Figures 1 and 2 present the uniformly weighted WSRT
km)configuration.Datawereacquiredindualcircularpo- and naturally weighted VLA contour maps of part of the
larizations with bandwidths of 25 MHz and at center fre- A2218 field, superimposed upon an HST F702W image
quenciesof8.1732,and8.2732GHz.Eachofthese25MHz (Kneib et al. 1996) of the region of sky that includes
IFswasfurtherdividedintosevenspectralchannelswitha the lensed sub-mm galaxy SMM J16359+6612 (Kneib et
width of 3.125 MHz. Observations were made assuming a al. 2004a). The positions of the nine sources detected by
coordinateequinoxof2000.Theabsolutefluxdensityscale SCUBA at 850 micron (Knudsen 2004) are also identi-
was set by observations of 3C 286. The data calibration fied as square boxes in these figures. Following Kneib
was made in the standard way with the phase variations et al. (2004a) we identify the lensed images as SMM-
during the observations calibrated via short observations A (SMMJ16359+6612.6), SMM-B (SMMJ16359+6612.4)
of the VLA calibrator source 1642+689. Unfortunately and SMM-C (SMMJ16358+6612.1).Both the WSRT and
observations of 1642+689 were only made 3 times over VLA observations detect radio emission associated with
the course of the 24 hour observations. The calibrated all three lensed images. The faintest SCUBA detection
A2218 data were clipped in order to remove a few high (SMM-C) is detected at the 4-sigma level by the VLA at
points before beginning the self-calibration process. The 8.2 GHz and at the 7-sigma level by the WSRT at 1.4
self-calibration process followed the same outline as that GHz.
described for the WSRT observations. The AIPS task IMFIT was used to fit Gaussian com-
ponents to the WSRT and VLA radio sources. Details of
AnaturallyweightedVLAimageofthefieldconvolved the radio source’s total flux density (ST), peak flux den-
with a Gaussian restoring beam of 11.2×10.4 arcseconds sity (SPk), position and (where appropriate) deconvolved
(in positionangle,PA=−72◦) is shownin Figure 2.The Gaussian sizes (major axis, minor axis and position an-
image has an r.m.s. noise level of 6 microJy beam. gle)aredetailedinTable1,togetherwiththeirassociated
Garrett et al.: Lensed radio emission in SMM J16359+6612 3
Table 1. Details of the WSRT and VLA radio sources associated with SMM J16359+6612 (SMM-A, B, C) and
SMM J16359+66118(arclet #289).
Name RA (+16h 35m) DEC (+66◦) ST SPk Maj Axis Min Axis PA
J2000 (sec) J2000 (′,′′) µJy µJy ′′ ′′ deg
WSRTJ163550+661205 (SMM-C) 51.1±0.1 12, 08.0±1.0 110±17 110±17 - - -
WSRTJ163553+661226 (SMM-B) 53.9±0.1 12, 26.4±0.6 391±43 217±16 15±2 9±2 165±12
WSRTJ163555+661237 (SMM-A) 55.6±0.2 12, 37.7±1.0 99±17 99±17 - - -
VLA J163550+661206 (SMM-C) 50.5±0.2 12, 06.8±0.1 24±10 24±6 - - -
VLA J163554+661223 (SMM-B) 54.5±0.1 12, 25.3±1.1 124±18 51±6 16±3 10±3 39±12
VLA J163555+661236 (SMM-A) 55.7±0.1 12, 37.1±0.5 28±6 28±6 - - -
WSRTJ163555+661152 (#289) 55.2±0.1 11, 52.9±0.6 170±29 175±17 6±4 - 43±20
VLA J163555+661150 (#289) 55.1±0.2 11, 50.1±0.6 35±10 37±6 8±3 - 65±10
TheSCUBA,VLAandWSRTobservationshavecom-
parable resolution. The error in the SCUBA 850 micron
positions (including confusion effects) is expected to be
∼ 4 arcseconds (Knudsen 2004). The radio positions are
expectedtobebetterthanthis,withanaccuracyof2arc-
secondorbetter.Slightlylargererrorsarepossibleforthe
radio positions derived by the WSRT, due to mild confu-
sioninthefield.Theradioandsub-mmpositionsofSMM-
A,B,C are all consistent within the errors - the largest
deviation is an offset of 3 arcseconds (the VLA/SCUBA
comparison in the case of the faintest image SMM-A).
There are no obvious systematic offsets in the sub-mm
and radio positions.
At 8.2 GHz the total flux density of SMM-B is 124
microJy, somewhat larger than our original expectations
(see section 1). By co-adding the total flux density of all
three images at 1.4 and 8.2 GHz, we derive a spectral in-
dex of the source α = −0.7, similar to the steep values
measured by Richards (2000) for star forming galaxies in
the Hubble Deep Field North. Kneib et al. (2004b) have
shown that the overall SED of SMMJ16359+6612is sim-
ilar to Arp 220. By adopting an Arp 220 SED, we esti-
matethesource’sk-correctedradioluminosityat1.4GHz
Fig.2. The VLA 8.2 GHz contour map superimposed
to be ∼ 4×1023 Watts/Hz. Using the relation between
upon an HST F702W image of the region of sky as-
Star formation rate (SFR) and radio luminosity (Condon
sociated with SMM J16359+6612F702W. Radio emis-
1992),wederiveanintrinsicstarformationrate(SFR)for
sion from all three lensed images is detected by the
WSRT. The boxes represent the SCUBA source posi- SMMJ16359+6612of500M⊙ yr−1.Thisisingoodagree-
ment with the value obtained by Kneibet al. (2004)from
tions. Radio emission from the highly magnified arclet
the FIR luminosity. As noted by Kneib et al. the uncor-
(#289), SMM J16359+66118 is also detected. Contours
are drawn at -3,3,4,7 & 10 times the 1-σ noise level of 6 rected Hα SFR estimate is only 11 M⊙ yr−1, suggesting
that this galaxy is highly obscured by dust.
microJy/beam.
TheVLA8.2GHzobservationsbestresolvethelensed
images. The flux density ratio of the images at 8.2 GHz
(andtheirformalerror)are∼0.23±0.06and∼0.19±0.08
formal errors. It was not possible to robustly constrain for SMM-A/SMM-B and SMM-C/SMM-B respectively.
the deconvolved size of the Gaussian component associ- Similarly, the WSRT 1.4 GHz flux image density ratios
ated with the fainter images SMM-A and SMM-C in the are 0.25 ± 0.05 and 0.28 ± 0.06 respectively. The radio
VLA and WSRT images. For SMM-A and SMM-C the flux density measurements at 1.4 and 8.2 GHz are thus
Guassian component size was thus fixed to the respective consistentwiththe sourcebeing gravitationallylensed.In
restoring beam sizes. For both the VLA and WSRT im- comparison,theSCUBAobservationsgivehighervaluesof
ages, two Gaussians were fitted to the SMM-A,B region 0.64 and 0.53, the IRAM CO(3-2) line intensities (Kneib
simultaneously. et al. 2004b)give 0.67 and 0.63,and the K-band observa-
4 Garrett et al.: Lensed radio emission in SMM J16359+6612
tions (Kneib et al. 2004a) give 0.53 and 0.4. Lens models SMG, SMMJ16359+6612. This is the first time that ra-
of this system (based on Kneib et al. 1996), predict flux dioemissionhasbeendetectedinamultiplyimagedSMG
density ratios of 0.63 and 0.4. Discrepancies in the flux systemlensed by a foregroundcluster.The maximumim-
density ratio at different wavelengths might be explained age separation is ∼ 41 arcseconds, much larger than any
if the various emission regions are not co-located or have other lens system detected in the radio. This is also the
verydifferentsize-scalesoverwhichthemagnificationmay firsttimethatmultiplylensedradioemissionhasbeende-
change. We also note that the flux density ratio of SMM- tectedfromastarforminggalaxy—allpreviousmultiply
A,BandCbetween850micronand1.4GHzare111±22, imaged radio lensed systems are associated with radio-
43±7and82±16.Bycomparisonthe850micron/1.4GHz loud AGN. The properties of the three radio sources are
flux density ratioofanotherSMG alsolocatedatz ∼2.5, largely consistent with the gravitational lensing hypoth-
SMMJ14011+0252(Ivisonetal.2000)is127±37.Itmay esis. In addition to detecting SMMJ16359+6612, we also
be that in the case of SMM J16359+6612,the discordant detectradioemissioncoincidentwithSMMJ16359+66118
flux ratios at both 1.4 and 8.2 GHz can be explained by a singly imaged arclet (#289). The sources are only de-
enhancedradioemissionassociatedwithSMM-B,perhaps tectablebycurrentradioinstrumentsduetothehighmag-
fromanotherradiosourcealsolocatedinthisregionofthe nification factor provided by the lens (∼45 in the case of
field. Higher resolution radio observations are required in SMMJ16359+6612).Theintrinsictotalfluxdensityofthe
order to settle this question. radio source at 1.4 and 8.2 GHz is ∼14 and ∼3 microJy.
As shown in Table 1, the brightest image SMM-B is
Follow-up radio observations of SMMJ16359+6612
resolvedbytheVLA8.2GHzobservations.Themeasured
with muchbetter angularresolutionthanthose presented
PA of the major axis is ∼ 39 degrees, in good agreement
here are warranted. Since sub-arcsecond imaging is rou-
with the position angle of the major axis of the CO(3-2)
tinely possible at radio wavelengths, it should be possi-
measurements of the same component, ∼ PA = 30 de-
ble to fully capitalise on A2218 as a “natural telescope”.
grees. Both measurements are consistent with the overall
A comparison with the detailed HST images of this sys-
extensionofthe associatedarc-likeHST images(Kneibet
tem will be particularly interesting. Measurements of the
al. 2004a). The FWHM of the major axis of the SMM-
size and a study of the morphology of the radio emit-
B (see Table 1) does not exceed ∼ 17 arcseconds. With
ting region should also constrain the extent of the star
a magnification of ∼ 22 for this image, the intrinsic size
formationin this galaxy.Upgraded radio telescopes (such
must be on the arcsecondor sub-arcsecondscale - consis-
as the EVLA and e-MERLIN) should be able to detect
tentwiththesub-galacticsizesmeasuredforradiosources
many more highly magnified lens systems lying behind
in the HDF-N.
massiveforegroundclusters.Inverydeepintegrations,the
The WSRT observations also detect other radio coun-
e-MERLIN and the EVLA telescopes now under develop-
terparts to other sub-mm sources detected by SCUBA
ment,maybeableto detectradiosourcesassociatedwith
in this field (Knudsen 2004). In particular, we detect ra-
individual SNe (or SNR), assuming these have luminosi-
dio counter-parts to SMMJ16357+66117 (4σ detection),
tiessimilartothosedetectedinlocalmergers,suchasArp
SMM J16361+66126 & SMM J16359+66118. The latter
220. Radio observations of SMMJ16359+6612 and other
source(seeTable1)ispreviouslyidentifiedbyKneibetal.
highly magnified SMG using existing and upgraded ra-
(1996)as a z=1.034singly-imagedlensed arc (alsoknown
dio telescope facilities, will be important in determining
as arc #289, see Swinbank et al. 2003 and references
the generalradioproperties(e.g.angularsize)ofthe faint
therein)withamagnificationof∼7.Thesourceisalsode-
SMG population, providing essential input to the techni-
tected by the 8.2 GHz VLA observations presented here,
cal specification of next generation instruments, such as
and by the ISOCAM at 15 micron (Barvainis, Antonucci
the Square Kilometre Array.
& Helou 1999). In both the WSRT and VLA images, the
measuredsizeofthearcletislessthantherestoringbeam,
inparticulartheminoraxesofthefittedGaussiansareun-
resolved.The position angle of the major axes are consis- Acknowledgements. We would like to thank the staff of
tentwiththeopticalextensionofthissystem.Thissource the WSRT including Rene Vermeulen, Raffaella Morganti
alsohasasteepspectralindex,α∼0.9.TheVLA8.2GHz & Willem Baan who helped make these observations pos-
observations do not detect the other SCUBA sources - sible at very short notice. We also thank the referee, J.-
SMMJ16357+66117 & SMM J16361+66126. These are P. Kneib for useful comments and suggestions, that have
fainter than SMM J16359+66118, and must have fairly improved the paper. The WSRT is operated by ASTRON
steep spectral indices (α>0.5). Further details of the ra- (The Netherlands Foundation for Research in Astronomy)
with support from the Netherlands Foundation for Scientific
diocounterpartstotheseSCUBAsourceswillbereported
Research(NWO).TheNationalRadioAstronomyObservatory
elsewhere.
is a facility of the National Science Foundation operated
under cooperative agreement by Associated Universities,
4. Conclusions Inc. This work was supported in part by the European
Communitys’s Sixth Framework Marie Curie Research
WehavedetectedwiththeWSRTat1.4GHzandVLAat TrainingNetworkProgramme,ContractNo.MRTN-CT-2004-
8.2 GHz, radio emission associated with the triply lensed 505183 “ANGLES”.
Garrett et al.: Lensed radio emission in SMM J16359+6612 5
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